At the other end of the table, 21 of the 35 firms evaluated now score at least three out of five stars, up from 19 companies out of 34 in the previous rankings.

The consumer group has called for Ofgem to tighten rules on who can get a licence to supply energy, crack down on poorly performing firms and reduce the cost of suppliers exiting the market – it claims supplier failures in the past year alone have left other customers having to pick up the tab for an estimated £160 million.

Energy UK’s CEO Lawrence Slade said: “In an increasingly competitive market, good customer service is just as important as price for many customers choosing their energy supplier.

“With 11 suppliers having exited the market since the start of last year – many of whom struggled with their customer service as well as financially – we agree that Ofgem needs to toughen the requirements for new and existing suppliers.”

Ian Cain, Managing Director of iSupply, said: “iSupplyEnergy was acquired by Vattenfall in 2017. Vattenfall is owned by the Swedish state and is one of Europe’s largest energy companies. We are well backed and here to stay.

“I was recruited late last year to lead an across the board improvement in how iSupplyEnergy serves its customers. With a new management team, our initial focus has been on resources, systems and processes.

“Since the turn of the year, these initiatives have been felt by customers. Our Trustpilot score has increased from less than 6/10 last year to 7/10 now, which Trustpilot rates as good. We will not rest there.

“Given the time period covered by the Citizens Advice Bureau rankings, these improvements are not yet reflected but will be as we progress through 2019.”

ELN has contacted Outfox The Market, Eversmart Energy and Pure Planet for a statement.